Generate objects with optional methods (Golang)

genOptIface.go

package main

import "fmt"

func genOptIface(name string, opt ...string) {
	if len(opt) == 0 {
		panic("zero opt types")
	}
	if len(opt) > 10 {
		panic("too many opt types")
	}

	iter := func(pre, iter, post func(int)) {
		for idx := 1; idx < 1<<len(opt); idx++ {
			if pre != nil {
				pre(idx)
			}
			for j := 0; j < len(opt); j++ {
				if 1<<j&idx != 0 {
					if iter != nil {
						iter(j)
					}
				}
			}
			if post != nil {
				post(idx)
			}
		}
	}

	fmt.Printf("func gen%s(types ...any) any {\n", name)

	iter(
		func(idx int) { fmt.Printf("type %s%d struct {\n", name, idx) },
		func(j int) { fmt.Println(opt[j]) },
		func(_ int) { fmt.Println("}") },
	)

	fmt.Println("var idx int")
	for _, t := range opt {
		fmt.Printf("var val%[1]s %[1]s\n", t)
	}
	fmt.Println(`
		for _, t := range types {
			switch t := t.(type) {
			default: panic("unknown type")
	`)
	for j, t := range opt {
		fmt.Printf(`
			case %[1]s:
				idx += 1<<%[2]d
				val%[1]s = t
		`, t, j)
	}
	fmt.Println(`
			}
		}
	`)

	fmt.Println(`
		switch idx {
		default: panic("unreacheable")
	`)
	iter(
		func(idx int) { fmt.Printf("case %[1]d: \n return %[2]s%[1]d{ \n", idx, name) },
		func(j int) { fmt.Printf("%[1]s: val%[1]s,\n", opt[j]) },
		func(_ int) { fmt.Println("}") },
	)
	fmt.Println(`
		}
	`)
}

func main() {
	genOptIface("x", "foo", "bar", "baz")
}